A conventional BTE (Behind-The-Ear) hearing aid comprises an ear hook that is bent so as to fit the ear, an earphone linked to the front end side of this ear hook, and a main body case linked to the rear end side of the ear hook. The ear hook has wiring that electrically connects the earphone and the main body case. An electrical connection part and a microphone are provided in that order from the front end side toward the rear end side inside the main body case. A front sound opening is provided to the front end side portion of the outer wall face of the main body case, and a rear sound opening is provided to the portion of the outer wall face of the main body case that is to the rear of the front sound opening (see Patent Literature 1 below, for example).
When this hearing aid is used, first the ear hook is hooked over the top of the ear, and then the earphone is placed against or inserted into the ear canal in this state. The main body case has a shape that hangs down while curving in a parabolic form from the front end side toward the rear end side thereof, so when the hearing aid is put on, this main body case is disposed along the rear side of the ear.